1. Field of the Invention
This invention relates in general to prestressed structural members and methods of forming such structural members, and more particularly, to a composite, prestressed structural member, such as a bridge unit, which has at least some precompression of the deck concrete in at least one direction and to methods of forming such a structure.
2. Description of the Prior Art
In the prior art there are a wide variety of structural members, both prefabricated and fabricated in place. These structural members include single element members, such as steel beams, and composite element members with molded materials reinforced with, or supported by, metal bars or support beams and elements. A typical molded material is concrete.
In forming structural members which include concrete or other moldable elements, or which are entirely made of concrete, it has often been found desirable to prestress the concrete to reduce tension loads therein. It is well known that concrete can withstand relatively high compression stresses but relatively low tension stresses. Accordingly, wherever concrete is to be placed in tension it has been found desirable to prestress the concrete structural member with a compression stress which remains in the structural member so that a failing tension stress is not normally incurred.
Conventional prestressing, as performed in the past, involves stretching a wire or cable through a mold and placing this cable in tension during hardening of concrete which has been poured into the mold. When the concrete has hardened the tension-loaded cable is cut, placing a compression load on the hardened concrete. The compression force from the severed cable remains with the element once it is removed from the mold.
A problem with conventional prestressing is that it requires careful calculations to avoid overstressing the cables because it is usually desirable to stretch the cables to near failure to achieve a sufficient prestressing. The apparatus necessary to achieve this prestressing is also complex. Further, cutting the cables can be a dangerous procedure and can ruin the prestressed structural member if not performed correctly.
In forming structural members for spanning between two supports, it has often been found desirable to utilize a steel structural support beneath a molded concrete surface. Because steel can withstand a much higher tensile stress, these composite structural members are formed with the steel sustaining most of the tensile stress which is placed on the member.
To form composite members of the type having an upper concrete surface and a metal structural support underneath, a metal piece form mold typically is utilized. First, the steel supports, such as wide flange beams, are placed beneath a mold assembly having two or more mold pieces disposed around the beam or beams. Next, the concrete is poured into the mold such that the concrete fills the mold and extends over the beam. When the concrete is hardened, the mold pieces are disassembled from around the beams such that the concrete rests on the beam. In most instances, these wide flange beam supported concrete structural members are formed in place. This is usually advantageous so the concrete surface can better fit into the finished structure. Some types of composite structural members, however, are prefabricated. The prestressing of such composite members may be carried out in a number of ways. One preferred method is disclosed in U.S. Pat. No. 4,493,177 in which the structure is formed in an inverted position.
A problem with large prefabricated structures is that they are difficult to move, and particular problems arise if the location is somewhat remote, as is frequently the case for bridge or building sites in developing countries. In these remote locations it is also difficult to utilize large cranes because of the difficulty in moving them to these locations. The present invention solves this problem by providing some bridge embodiments which are easily constructed at the desired location by using relatively small prefabricated panels or composite units which are transversely attached to a plurality of longitudinally extending girders. When the structure is in position, the concrete portion thereof is substantially always in compression. By using fewer longitudinal girders to support the bridge, the present invention also reduces the total weight of structural steel required.
Reduction in the weight of structural steel is also accomplished by the reverse stressing of the girders as they are loaded with the composite units. The bottom flange of each girder, which will have tensile stress when the structure is in its final position, receives and retains at least some compressive stress during the construction process. This prestressing of the girders allows reduction of their weight.